The latest CDC report . . . found that there were about 50,000 new infections each year from 2006-2009.

Gay and bisexual men account for the majority of the new cases — nearly two-thirds in 2009. The only increase over the four years was in young gay and bisexual men, driven by a 48 percent increase among blacks ages 13 to 29.

The annual number of new infections peaked at 130,000 in the mid-1980s, said Dr. Kevin Fenton, the AIDS prevention chief for the CDC. He said the report presented a mixed picture. While new infections have plateaued, "stability is not acceptable" and the increase in young men is worrisome, he said.

The American epidemic is still concentrated primarily in gay men, and is growing rapidly worse among young black gay men.

For most risk groups, infection rates are stable at 61 percent through gay or bisexual sex, 27 percent through heterosexual sex and 9 percent through drug injection.

That realization is causing a rift in the AIDS community. Activists say the persistent H.I.V. infection rate proves that government prevention policy is a flop. Federal officials are on the defensive even as they concede that the epidemic will grow if prevention doesn’t get better, which they know is unlikely while their budgets are being cut.

And some researchers believe it is impossible to completely wipe out even a fatal, incurable disease when it is transmitted through sex and carries so much stigma that people deny having it and avoid being tested for it.